The correct answer is option 'C' - Lime-sand mixture.

Explanation:
Effective earthing is crucial for the safety and proper functioning of electrical systems. It helps to dissipate fault currents, prevent electric shocks, and protect equipment from damage. The choice of mixture for filling around the earth electrode is important as it affects the conductivity and stability of the earthing system.

Here are the reasons why lime-sand mixture is preferred for filling around the earth electrode:

1. Conductivity: Lime-sand mixture has good electrical conductivity, which is essential for effective earthing. It allows the fault currents to flow efficiently through the earth electrode and dissipate into the ground.

2. Moisture retention: Lime-sand mixture has the ability to retain moisture, which helps in maintaining a low resistance path for current flow. Moisture is important for reducing the resistance between the earth electrode and the surrounding soil, ensuring effective earthing.

3. Chemical resistance: Lime has high chemical resistance, making it suitable for use in various types of soil conditions. It does not easily degrade or react with different soil compositions, ensuring long-term stability of the earthing system.

4. Mechanical stability: Lime-sand mixture provides good mechanical stability to the earthing system. It prevents the earth electrode from shifting or moving due to external factors such as wind, vibration, or soil settlement. This stability is important to maintain the integrity of the earthing system over time.

5. Cost-effectiveness: Lime-sand mixture is relatively inexpensive compared to other mixtures such as coal-salt or pepper-salt. It provides a cost-effective solution for filling around the earth electrode without compromising on the performance of the earthing system.

In conclusion, lime-sand mixture is preferred for filling around the earth electrode for effective earthing due to its good electrical conductivity, moisture retention capability, chemical resistance, mechanical stability, and cost-effectiveness. It ensures a low-resistance path for fault currents, maintains the stability of the earth electrode, and provides long-term performance of the earthing system.